The present invention relates to a photoconductor for electrophotography adapted for use in electrophotographic apparatuses operating at high speeds and at high resolutions, such as high-speed and high-resolution printers, copying machines and facsimiles. The present invention also relates to a method of manufacturing and using such a photoconductor.
To date, tremendous efforts have been focused on improvements in the printing speed, image quality, and resolution of electrophotographic apparatuses, such as copying machines, printers and facsimiles. For conventional electrophotographic apparatuses with printing speeds between 40 and 100 pages per minute and with resolutions of 240 dpi or less, photoconductors that use As.sub.2 Se.sub.3 as the photoconductive material have been widely adopted by virtue of their excellent resistance against wear after repeated printing cycles. Typically, the thickness of the photoconductive layer is adjusted to be from 60 to 80 .mu.m, since this layer thickness has been found to reduce the occurrence of image defects when the photoconductor is charged during image development using an electric potential of around 1,000 V.
FIG. 1 is a schematic diagram illustrating a typical imaging process in an electrophotographic apparatus. As shown in FIG. 1, a photoconductor 10 is charged in an charging section 1 in the dark. In an exposure section 2, the photoconductor 10 is exposed to light in a pattern corresponding to the image to be produced. The exposure to light causes a latent electrostatic image to be formed on the photoconductor surface. In a development section 3, developing powder is deposited on the latent electrostatic image, forming a "developed" image. The "developed" image is then transferred onto a carrier paper 6 in a transfer section 4, and the transferred image is fixed onto the carrier paper 6 in a fixing section 5.
FIG. 2 is a cross-sectional schematic diagram showing a photoconductor being charged and then exposed to light. As shown in FIG. 2, a photoconductor 10 includes a photoconductive layer 20 formed on a conductive substrate 30. The photoconductive layer 20 is charged in a charging section 1 under a high voltage (HV). The charging section 1 produces positive charges 12 on the surface of the photoconductive layer 20. When the photoconductor is exposed to light, however, positive and negative charge carriers 14 and 16, respectively, are generated within the photoconductor. Because of the presence of an electric field in the photoconductive layer 20, the positive charge carriers 14 migrate toward the conductive substrate 30, and the negative charge carriers 16 migrate toward the surface of the photoconductive layer 20. When the negative charge carriers 16 reach the surface of the photoconductive layer 20, they neutralize the positive charges 12 thereon, thereby reducing the electric potential of the photoconductor surface. The period of time between when the photoconductor is first exposed to light and when the potential of the photoconductive layer surface drops is determined by the migration period of the negative charge carriers. This period measures the optical response of the photoconductor and hereinafter will be referred to as the "potential drop period."
The potential drop period has consequences for the maximum speed at which an electrophotographic apparatus is able to operate. As the speed of forming the latent electrostatic image is increased--that is, as the rotating speed of the photoconductor is increased--the quantity of light radiated onto the photoconductor surface is reduced. Therefore, to achieve the same reduction in electric potential, the photoconductor is required to exhibit higher photo-sensitivity. Since it takes a certain period of time for the potential of the photoconductor surface to reach its lower level after the photoconductor surface is exposed to light, if the photoconductor does not have increased photosensitivity, when the interval between the light exposure and development steps is shortened (which is the case when the speed of operation of the electrophotographic apparatus is increased), the development step starts before the electric potential of the photoconductor is sufficiently reduced. This unwanted early start of the development step causes imaging defects to occur, such as undesirable density distributions in the developed image. In short, as the speed of operation of an electrophotographic apparatus increases, the photoconductor used therein is required to exhibit an improved optical response to maintain a high image quality.
Although the effect of the potential drop period may be compensated for by increasing the outer diameter of a photoconductor, the outer diameter has an upper limit determined by the outer dimensions of the electrophotographic apparatus in which the photoconductor is used.
Another approach for meeting the requirements of high image quality has been to produce fine-grained developing powder to improve image resolution. However, since the conventional photoconductive layer is thick, some generated carriers migrate laterally. The lateral carrier migration causes bleeding and blurred images. Thus far, however, it has not been possible to form a thin photoconductive layer on a conductive substrate machined by cutting. A conductive substrate machined by cutting typically has a surface roughness Rmax of 0.8 to 12 .mu.m, which is not desirable for obtaining a thin photoconductive layer. The burrs produced by machine cutting cause voids and black spots in images.